Why did the Apollo 13 astronauts feel as if they were suffocating, despite having an adequate amount of oxygen?

Even though sufficient O2 in exhaled air exists to re-breathe, CO2 % rise & low blood pH results in a feeling of suffocating.

They were re-breathing the air they exhaled. This will cause higher CO2 levels even though there is enough O2 in exhaled air for you to re-breathe. pH would be low because of the increased CO2 levels from re-breathing the exhaled air. It takes high CO2 level and low blood pH to feel like you are suffocating.